The discovery of new high T.sub.c superconducting materials in the bismuth strontium calcium copper oxide (Bi-Sr-Ca-Cu-O) system represented a step forward in oxide superconducting materials. This system also includes formulations including lead (Bi-Pb-Sr-Ca-Cu-O). The bismuth strontium calcium copper oxide system contains three major phases with different critical temperatures (Tc's): a higher T.sub.c "2223" phase, Tc=110K; a lower T.sub.c "2212" phase, T.sub.c =80K; and a very low T.sub.c "2201" phase, T.sub.c =20K. As used herein, the term "2223 phase" denotes a phase having the nominal formula or (Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x (Bi,Pb).sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x ; the term "2212 phase", a phase having the nominal formula Bi.sub.2 Sr.sub.2 Ca.sub.1 Cu.sub.2 O.sub.x ; the term "2201 phase", a phase having the nominal formula Bi.sub.2 Sr.sub.2 Cu.sub.1 O.sub.x.
The crystal structures of the phases are characterized by c-axis lattice constants of about 36.8 .ANG., about 30.6 .ANG., and about 24.2 .ANG. respectively. Since the phases differ from one another in the number of Cu-O and Ca layers per unit cell, syntactic intergrowths can occur through faults in the stacking sequences. These intergrowths interfere with efforts to obtain pure superconducting single crystals. Efforts aimed at direct crystal growth of pure 2223 phase from the melt or flux have been unsuccessful.
Formation of the higher T.sub.c 2223 phase, in a volume fraction sufficient to materially affect the bulk T.sub.c, has been achieved in these materials only by conversion of the lower T.sub.c 2212 phase to the 2223 phase by heating samples at high temperatures near the melting temperature for long periods of time, measured in days or weeks. A variety of ceramic processing methods have been used to increase the fraction of the higher T.sub.c phase. These include (a) addition of lead; (b) use of copper- and calcium-rich starting compositions; and (c) prolonged heat treatment close to the melting point. The substitution of lead for bismuth in the bismuth oxide planes of the crystal structure of the 2223 phase has been used to strengthen Bi-O bondings and to facilitate the formation of a relatively pure 2223 phase. Furthermore, the addition of lead oxides into the matrix also can lower the melting point and viscosity of the glassy (amorphous) phase, probably by acting as a fluxing agent. All of these effects suggest that the formation of 2223 phase is a thermodynamically difficult and kinetically sluggish process.
Even when using methods a, b, and c, a long annealing time is usually still required to promote formation of the high T.sub.c phase. For example, method (a) can involve annealing at 845.degree. C. for 200-300 hr to form samples containing 90% of 2223 phase; method (b) can involve a rapidly solidified melting process followed by an annealing at 865.degree. C. for 11 days to produce a mixture of 2212 and 2223 phases; and method (c) can involve annealing at high temperatures for 18-26 days to produce 50-90% of 2223 phase. Such long annealing times severely affect the fabrication of the Bi-Sr-Ca-Cu-O superconductors into appropriate shapes, such as wires and tapes, for practical applications.